The present invention relates to a method of bonding an electronically conductive interconnection layer on an electrode of an electrochemical cell.
High temperature electrochemical cells are taught in U.S. Pat. No. 4,490,444 (Isenberg). In these type of cells, typified by fuel cells, a porous support tube of calcia stabilized zirconia, has an air electrode cathode deposited on it. The air electrode may be made of, for example, doped oxides of the perovskite family, such as lanthanum manganite. Surrounding the major portion of the outer periphery of the air electrode is a layer of gas-tight solid electrolyte, usually yttria stabilized zirconia. A selected radial segment of the air electrode is covered by an interconnection material. The interconnection material may be made of a doped lanthanum chromite film. The generally used dopant is Mg, although Ca and Sr have also been suggested.
Both the electrolyte and interconnect material are applied on top of the air electrode by a modified electrochemical vapor deposition process, at temperatures of up to 1450.degree. C., with the suggested use of vaporized halides of zirconium and yttrium for the electrolyte, and vaporized halides of lanthanum, chromium, magnesium, calcium or strontium for the interconnection material, as taught in U.S. Pat. No. 4,609,562 (Isenberg et al.). A nickel zirconia cermet fuel electrode, which is applied on top of the electrolyte is also attached by electrochemical vapor deposition; where nickel particles are anchored to the electrolyte surface by a vapor deposited skeleton of electrolyte material, as taught in U.S. Pat. No. 4,582,766, (Isenberg et al.).
Magnesium is the primary dopant used for the interconnection material. However, magnesium doped lanthanum chromite, for example La.sub..97 Mg.sub..03 CrO.sub.3, has 12% to 14% thermal expansion mismatch with the air electrode and electrolyte material. Adequately doping the LaCrO.sub.3 interconnect material with Ca or Sr or Co or their combination would eliminate the thermal expansion mismatch problem with the air electrode and electrolyte material. It has been found, however, that there are certain thermodynamic limitations in adequately doping the interconnection with Ca, Sr or Co from a vapor phase by the electrochemical vapor deposition process at temperatures exceeding 900.degree. C.
U.S. Pat. No. 4,631,238 (Ruka), in an attempt to solve potential interconnection thermal expansion mismatch problems between the interconnect, electrolyte, electrode, and support materials, taught cobalt doped lanthanum chromite, preferably also doped with magnesium, for example LaCr.sub..93 Mg.sub..03 Co.sub..04 O.sub.3, as a vapor deposited interconnection material using chloride vapors of lanthanum, chromium, magnesium, and cobalt.
U.S. Pat. No. 4,895,576 (Pal et al.), taught forming a layer of particles, selected from CaO, CaO.sub.2, SrO, SrO.sub.2, CoO, Co.sub.2 O.sub.3, BaO, BaO.sub.2, MgO, or MgO.sub.2, on the interconnection portion of a fuel cell air electrode, heating the structure, and then vapor depositing a skeletal structure of lanthanum chromite around and between the metal oxide particles. The metal ions of the incorporated metal oxide particles diffuse into the bulk of the lanthanum chromite structure when annealed at higher temperatures. At the end of the process, there is a complete disappearance of the discrete metal oxide particles and it becomes a doped lanthanum chromium oxide structure. This process requires an additional long term annealing step, at about 1,300.degree. C., to maximize conductivity by distributing the dopant across the lanthanum chromium oxide film. Also, since the dopant distribution is diffusion controlled, there is a gradient in the dopant concentration across the film cross section and the direction of diffusion is not easily controlled. Some of the dopant can also diffuse into the air electrode.
U.S. Pat. No. 4,861,345 (Bowker et al.) taught sintering particles of LaCrO.sub.3, doped with Sr, Mg, Ca, Ba or Co and coated with calcium oxide or chromium oxide, at 1400.degree. C. Here, the coatings on the particles help in sintering, by providing a liquid phase, and the cations present in these coatings get absorbed into the LaCrO.sub.3 structure. However, in this process, sintering the particles to make a leak tight interconnection film and then bonding it to the air electrode can be problems.
None of the proposed solutions solve all the problems of thermal expansion mismatch, and, problems associated with doping calcium, strontium, cobalt, and barium by vapor deposition, or of providing a uniformly thick, leak tight, well bonded interconnection in a simple and economical fashion. It is an object of this invention to solve such problems.